Exploring Donor-Functionalized Carbenes and their Derivatives for Small Molecule Activation and Catalysis

Baguli, Sudip (2024) Exploring Donor-Functionalized Carbenes and their Derivatives for Small Molecule Activation and Catalysis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sudip Baguli (19RS057)) 19RS057.pdf - Submitted Version Restricted to Repository staff only Download (51MB)

Abstract

Ligand design has always been a major aspect in synthetic organometallic chemistry and catalysis. In this regard, hybrid ligands are increasingly more prevalent due to their notably distinct chemical functionalities and coordination pattern, leading to a wide range of reactivities and catalytic activities. On a different note, carbenes have emerged as highly potent neutral 2e- donor phosphine analogues with weak to moderate -accepting abilities, which are also widely being considered in constructing hybrid ligands. The two most influential cyclic aza-carbenes are the NHCs and CAACs; the first one is relatively more classic and fairly well-explored compared to the latter one. In this thesis, new donor-functionalized CAACs and NHCs and their derivative-based hybrid ligands are primarily synthesised, and their reactivities and catalytic activities are investigated in the context of the main group and transition metal organometallic chemistry. The pioneering Bertrand’s group has recently reported CAACs with hemilabile sidearms like ether, amine, imine, and phosphine. The imine derivative, in particular, has been further explored, which reveals an upper hand in catalysis compared to non-functionalized CAACs. We, in parallel, are investigating picolyl-CAACs with the possibility of placing the picolyl group both at the C- and N- side, which could potentially be bidentate hybrid ligands leading to flexible six-membered metallacycles upon chelation. Some intriguing discoveries in the field of organic chemistry have been made during the synthesis of picolyl-tethered ‘C’ sided and ‘N’ sided CAACs through unconventional carbene synthetic routes. For example, external Lewis acid catalyst-free water activation by a donor-acceptor cyclopropane, which was made from the ‘C’ sided picolyl-tethered CAAC ligand precursor by virtue of an acidity battle between picolyl and pyrrolinium protons. In the presence of carbene trapping agents like CuCl and Se, carbene can be generated from deactivated carbene species, such as from DA cyclopropane, through Retro C(sp³)−H activation and also from a 2° alcohol through 1,1-dehydration. Extensive research has been conducted on the isolation of thermally stable picolyl-based azomethine ylides from ‘N’- sided picolyl-CAAC precursors and their unique reactivities towards the formation of carbenes in the presence of CuCl. In parallel, its FLP-like reactivity toward the small molecules is also extensively investigated. It easily activates the nonpolar B-B bond of B₂Pin₂ and polar B−H, O−H, Si−H, Al−H and C−I bonds. Also, it could easily do the CS₂ fixation at ambient conditions. Four new cationic Rh(I) complexes of carbene and their derivatives, which include picolyl-linked cyclic amide, selenamide, aziridine, and CAAC-based ligands, were tested for their excellent catalytic activities in the areas of hydrogenation and hydrosilylation of C=C and C≡C bonds. Three new (CAAC)CuCl complexes [(ArCH2,MeCAAC)CuCl] (Ar = Ph, 1-naphthyl, 1-prenyl) are synthesised. Their carbonyl and ester hydrosilylation catalytic activities were investigated; as a reference, the parent [(Me2CAAC)CuCl] system was taken into consideration, where the effective steric around the copper centre practically remains the same for all the cases, and all three are equally active in carbonyl and ester hydrosilylation as [(Me2CAAC)CuCl]. For clarification on the role of a “(CAAC)CuH”, “(PhCH2,MeCAAC)CuH” is generated in situ and is trapped as its BH₃ adduct (PhCH2,MeCAAC)CuBH₄. Another interesting finding is there with Au⁽I⁾ and Cu⁽I⁾ precursors towards the aryl-iodide functionalized CAAC and NHC ligand systems. Lastly, a new open-chain hybrid and flexible ligand was synthesised by doubling the fluorenyl-tethered-NHC moiety using a -CH2- linker, that offers four sequential organic donor modules and enwraps a Ca²⁺ pseudo-tetrahedrally in a twisted ‘S’-shape is devised. The [(L)Ca] synthesis is intriguingly stepwise involving the intermediate [(LH)Ca(HMDS)], whereas [(LH)Li₂(HMDS)] shows the potential for bimetallic chemistry.

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